1. Field of the Invention
The invention relates to an electromagnetic actuating device, in particular for a valve, includes a plunger which is axially movable between two extreme positions, and an electromagnetic coil for actuation of the plunger, the coil surrounding the plunger, and a permanent magnet to hold the plunger in one of its extreme positions, and at least one pole plate located between the plunger and the permanent magnet.
2. The Prior Art
In EP-A 373 142 a bistable magnet is described which is provided with a permanent magnet holding the actuating plunger in one of its extreme positions. On the permanent magnet a guide plate is placed whose diameter is smaller than that of the magnet. The plunger may be brought into contact with this guide plate by means of a magnetic short-circuit plate.
In this known arrangement a comparatively large permanent magnet of a conventional alloy, such as AlNiCo, is provided.
The demand for miniaturization of all components and the progress made in developing magnetic alloys have led to an increasing use of permanent magnets based on rare earths. The energy density of both the magnets based on samarium-cobalt and those of the neodymium-iron-boron group is considerably higher than that of conventional magnets. The magnetic properties of these materials are utilized best if the magnets are given a flat, disk-like shape.
It has been found that in known actuating devices of the above kind the magnetic holding forces obtained with the use of high-grade permanent magnets have proved unsatisfactory.